There's no doubt that Bob Justman's involvement was crucial to the first season of TNG. He was the one calling for a Klingon on the bridge, and he was the one pushing for Patrick Stewart. And those are just the big things that we know about. Justman's attention to detail throughout his career tells me that there were a thousand things he watched over on that show, for the time that he was there.
David Gerrold's contribution to TNG was frankly that Roddenberry cribbed some of the best ideas from "The World of Star Trek" and had Gerrold throw them into the series bible. Some of the scenes in the pilot are clearly Gerrold ideas - particularly the dialogue between Picard and Riker about whether the captain can beam down. I would have liked to have seen the episodes that Gerrold could have written himself, and I wish that Roddenberry hadn't blocked Gerrold's "Blood and Fire" as he did. I frankly think Gerrold was badly treated by Roddenberry, as was Fontana. I wish that hadn't happened. The result was that we lost both Gerrold and Fontana from the series very early on, and things wound up in the legal arena. (Gerrold had a solid legal point that much of the foundation of TNG was from his concepts, granted that this was within the overall world that the original series had created.)
The other thing is that the first season of TNG was the only one where Roddenberry was really that heavily involved, as was his attorney. Following that, he stepped back and acted more as a consultant during the final years of his life. From what I've seen, his input was helpful up through the 3rd season, after which he was almost completely disengaged. (There's a great story from the 3rd season about "Deja Q" that the writer had originally planned it that Q was faking his mortality. Roddenberry famously and correctly challenged the writers, saying that this would be a false narrative - if you want to tell the story about the god who fell to Earth, tell that story and see where it takes you. The result was a really interesting (and funny) episode, and a good piece of Star Trek writing.)